This invention relates to air brakes for wheeled vehicles such as trucks, tractors, trailers and buses, and particularly is directed to anti-lock air brake systems for minimizing the possibility of brake lock-up and the resultant skidding that may occur as a consequence, and possible jack-knifing that may occur, as between a tractor and trailer.
This invention more particularly is directed to an anti-lock air brake system that may either be built-in or interposed as a retrofit system in an existing vehicle air brake system between the relay valve, which distributes air through air service lines to the air-actuated diaphragms or brake actuators for each of the brakes in the vehicle, and the air-actuated diaphragms or brake actuators, which cause the brake shoes associated with such air-actuated diaphragms or brake actuators to be moved into frictional engagement with the associated brake drums to bring a wheeled vehicle to a controlled stop.
There are a number of anti-lock air brake systems in the prior art, such as U.S. Pat. Nos. 3,268,271; 3,545,818; 3,976,335; 3,976,336; 3,977,734; 4,035,034; 4,372,620; 4,538,859; and 4,708,403.
U.S. Pat. Nos. 3,976,336 and 4,035,034, mentioned above, depend upon the use of wheel sensors to sense slip rate of the wheels, and the use of computers to produce a throttling electric signal (a) to solenoid air control valves before the braking pressure reaches a certain pressure increase target value or (b) when the braking pressure is drawing near a certain pressure decrease target value, so as to maintain a certain specified slip rate. U.S. Pat. No. 4,372,620 is another example of the use of wheel sensors. U.S. Pat. No. 4,538,859 employs a pendulum that serves to select the mode of braking, such as intermittent or conventional.
U.S. Pat. Nos. 3,268,271 and 3,545,818, mentioned above, disclose details of typical cyclic air valves for use in preventing wheel skid, while U.S. Pat. No. 4,708,403 discloses an air valve control which provides rapid OFF-and-ON impulses manually without the use of electrical or electronic controls, and which is dependent upon the amount of force applied by the operator against the brake pedal. U.S. Pat. Nos. 3,976,335 and 3,977,734 disclose other valve structures used in controlling braking pressure.
The present invention differs over the prior art in that there are not computers or wheel sensors involved. One significant difference is that the anti-locking braking system is initiated each and every time the operator applies the brakes, either by use of the foot-actuated valve, which in the instance of a tractor and a trailer, for example, controls the brakes for both the tractor and trailer, or by use of the hand-actuated valve for controlling only the brakes of the trailer. If, however, the operator only taps the foot-actuated valve so that the braking pressure does not exceed 10 pounds (psi), then the anti-locking braking system is not initiated.
Some advantages resulting from the operator initiation of the anti-locking system of the present invention over the prior art include, in addition to being preventive against lock-up of the brakes, a saving on brake wear due to the momentary cooling that occurs when the brakes are momentarily released during the hereinafter-described pulsing sequence. There is also a saving on tires since the tires will not be flat-spotted as a consequence of locking and thereafter dragging them, thus undesirably wearing inches of rubber off the tires where they contact the roadway.
The present invention takes into consideration the realities of truck, trailer and bus operation. As a practical matter, there are often a certain amount of mechanical deficiencies present, such as differences in adjustments between the slack adjuster arm on the brake actuator on each end of the same axle for a truck or trailer or bus, and differences in the radial distances brake shoes have to travel before coming into full frictional engagement with the drums of the associated wheels. There can also be differences in drum diameters. Further, one tire may be more worn than a tire at the opposite end of the same axle. Consider, for instance, what happens when one wheel on an axle has a drum having a greater circumference than the drum of the other wheel on the same axle. The greater-circumferenced drum causes the associated wheel to want to travel farther than the smaller-circumferenced drum and associated wheel, with the end result that this tends to cause the vehicle or trailer to turn toward the direction of the drum with the smaller circumference. The anti-locking braking system of the present invention tends to compensate for differences such as these by providing each time of operator brake initiation a momentary release of the brakes during the hereinafter-described pulsing sequence, which, in the instance of a smaller-circumferenced drum or worn tire on one end of an axle, allows the smaller-circumferenced drum or worn tire to speed up. The result is that there is a tendency to equalize the paths of the wheels on the same axle to keep them traveling in a straight-line direction and thereby bring the wheeled vehicle to a safe stop.
The timing of the momentary release of the brakes, as will hereinafter be described, is such as to allow only a slight release of the air-actuated diaphragms, which control the movement of the brake shoes into frictional engagement with the brake drum. This tends to make application of the brakes smoother because the distances traveled by the slack adjuster arm, the rotation of the S-cam for the brake shoes and the radial travel for engagement of the brake shoes carrying the frictional material all act as though they are in mechanical balance at all times. This occurs because of the fact that the control is initiated simultaneously on each wheel instead of only affecting the locking wheel, as would be the case in the prior art with use of wheel sensors. This resulting mechanical balance, therefore, tends to cause the vehicle to stay in a straight line upon braking to a stop. There will also be less steering input required on the part of the operator.
Still another advantage over the prior art: when driving around a curve in the road with one wheel traveling faster than the other on the same axle, the present invention does not sense a brake lock-up in fact, as sometimes occurs with the use of wheel sensors. The wheel sensor falsely senses that a slower-rotating wheel is locking up and thus undesirably causes more braking force to be applied to the faster-rotating wheel and at the same time releases braking force on the slower rotating wheel. This causes an unstable condition that tends to drag a trailer, for instance, out into the other lane of traffic. The invention, therefore, ignores differences in circumferential speed and thus maintains stability in control of the vehicle.